1. Field of the Invention
The present invention relates generally to digital amplifiers and more specifically to digital AGC amplifiers having a large AGC range capability to correct for large amplitude variations in input signal level, whether occurring rapidly or slowly, while simultaneously preserving at the amplifier output a low level, low frequency modulation, such as amplitude modulation, on the input signal. The present invention also relates to receiver circuitry particularly suitable for use in satellite communications systems wherein ionospheric disturbances, such as ionospheric scintillation, cause large and rapid signal fades which must be gain controlled out, while simultaneously preserving the low level low frequency dither signal from the satellite tracking antenna. The present invention also relates generally to analog-to-analog or analog-to-digital conversion circuitry wherein large variations in input signal level are rapidly corrected while small input signal level variations within a predetermined range are preserved.
2. Description of the Prior Art
A variety of digital AGC amplification techniques are present in the prior art for providing rapid automatic gain control for fluctuation in input signal level. However, in certain applications, a low level low frequency variation in the power level must be preserved at the amplifier output. The preservation of such a low level modulation is critical in satellite communications in which rapid power fades occur concurrently while tracking the satellite with a high gain disk antenna, dithered on the target at a low frequency, i.e. 10 Hertz, which dithering produces the aforementioned low frequency power amplitude variation. The antenna control signal, at the AGC amplifier input, must pass through the amplifier to the antenna control system and be preserved while gain controlling out large random signal level variations. Amplifiers of the known prior art, while effectively smoothing large fluctuations in the input signal level, do not preserve the aforementioned low level low frequency modulation at the amplifier output, while simultaneously providing rapid AGC.
Representative of the prior art as characterized hereinabove, U.S. Pat. No. 3,434,065 describes an AGC amplifier wherein all input power variations within the amplifiers dynamic range are tracked out once the controlled output level reaches a threshold value. U.S. Pat. No. 3,683,367 describes a digital AGC amplifier for use with input data signals which dwell on either a maximum or minimum level for a significant time duration. U.S. Pat. No. 3,879,672 describes a digital AGC circuit for providing rapid gain changes in response to rapid changes in input signal amplitude. U.S. Pat. No. 3,898,580 describes a dual time constant phase lock oscillator including a stepwise gain variable operational amplifier.